1. Field of the Invention
The present invention relates to illuminating optical apparatus and exposure apparatus well suited for the manufacture of semiconductor devices.
2. Description of the Prior Art
Up to date, reduction projection-type exposure apparatus have been used in the manufacture of semiconductor devices which are composed of very fine patterns such as LSIs and VLSIs and much efforts have been made for the transfer of finer patterns. To cope with such trend toward the realization of finer patterns, attempts have been made to increase the numerical aperture (hereinafter referred to as an NA) of a projection optical system along with a reduction in the wavelength of an exposure light and projection optical systems of over NA=0.5 have been realized.
In the actual projection exposure using a projection optical system having such a large NA, depending on the minimum line width, etc., of a pattern to be transferred, it is important to optimize the NA of the projection optical system and the illumination condition. This illumination condition is determined by adjusting a .sigma. value (referred to as a so-called coherent factor) which corresponds to the ratio of the NA of the illuminating optical system to the NA of the projection optical system, and to adjust the ratio between the NAs of the two optical systems so as to attain the Proper balance between the resolving power and contrast of a given pattern has been proposed for example in Japanese Laid-Open Patent No. 59-155843, etc.
According to the proposed apparatus, a variable aperture stop having a variable opening is arranged at a position on the exit side of a fly-eye lens where secondary light source images are formed in an illuminating optical system and the size of the opening of the variable aperture stop is varied, thereby controlling the size of the light source images. In other words, since the .sigma. value corresponds to the ratio between the sizes of the pupils of the projection optical systems, the degree of light shield for the actually formed light source image is varied so that the size of the light source images is varied and thus the NA of the illuminating optical system is substantially varied. Thus, in accordance with the fineness of a pattern to be projected and exposed, the optimization of the illumination condition or the optimization of the .sigma. value is sought so as to attain the optimum condition between the resolving power and contrast of the given pattern.
Generally, with this type of exposure illuminating apparatus, an illumination of a higher illuminance is required on a surface to be irradiated (a mask or a reticle) in order to improve the throughput during the exposure and printing of fine patterns. With the previously mentioned conventional apparatus, however, to ensure that the optimization of the .sigma. value or the illumination condition is accomplished to ensure the optimum conditions for the resolving power and contrast of a given pattern in accordance with the fineness of the pattern to be projected and exposed, the size of the opening of the variable stop is adjusted. Therefore, if the opening is decreased, the peripheral portion of secondary light source images is blocked by the variable stop thus causing a loss in the light quantity.
In other words, since all the light beam from the light source is irradiated when the .sigma. value is increased to a maximum (when the diameter of the variable aperture stop is made maximum), the maximum illuminance is obtained on the surface to be irradiated (the mask or the reticle). When the .sigma. value is made smaller than the maximum value (when the diameter of the variable aperture stop is decreased), however, a part of the light beams from the secondary light source images is blocked by the variable stop and thus the total light quantity of the irradiated light is decreased and the illuminance on the irradiated surface is decreased. As a result, there is the fatal disadvantage that the exposure time required for the proper exposure is increased and consequently the throughput of the exposure operation is inevitably deteriorated.